In the oil and gas industry, it is well known to make measurements along a wellbore using a tool which is raised or lowered in the wellbore suspended from a line or cable such as a wireline or a slickline. Such measurements are often referred to as well logging and may relate to a property of an environment in or adjacent to the wellbore or to a property of an object located in or adjacent to the wellbore. For the purposes of analysis of data measured using a well logging tool, it is important to accurately associate the true depth of the tool with the corresponding data measured by the tool at the true depth.
In a typical logging system, the length of line unspooled at the surface is measured and the depth of the logging tool in the well is assumed to be equal to the measured length of line unspooled. The length of line unspooled is usually determined by placing a measurement sheave wheel of known diameter in contact with the line and counting the number of revolutions of the measurement sheave wheel. However, the accuracy of the measured unspooled line lengths thus obtained may depend on line elasticity and line tension. Line tension may vary according to static and/or dynamic forces which act on the tool as the tool is lowered or raised in the wellbore. The main static forces are line weight and tool weight. The dynamic forces may include friction or fluid drag forces on the line and the downhole tool. Accordingly, it is known to correct the determined tool depth for line tension. The line tension is usually measured at surface by deflecting the line over one or more sheaves or pulleys and measuring the resultant force on a shaft of a sheave or pulley.
It is also known to employ other methods and tools for correction of the measured tool depth. One method defines the tool position by correlation to well or casing geometry by using a magnetic casing collar locator (CCL) or a gamma ray type sensor. Other methods make use of magnetic marks placed on the line used to lower or raise the downhole tool in the wellbore.
Typically, logging measurements are made using downhole sensors at regular intervals as the tool is raised by the line at a fixed speed. For logging operations performed using an electrically conductive line such as a wireline, the sensor data may be transmitted to the surface through one or more electrical conductors of the line. However, wireline logging operations are generally more costly than slickline logging operations because wireline logging operations generally require a more complex sealing arrangement between the wellhead and the wireline for containing well pressure. Accordingly, it is known to perform logging operations using a slickline which allows a seal to be more easily created at the wellhead. For logging operations performed using slickline, the sensor data is usually recorded to a memory of the tool. It is also known to perform logging operations using an insulated slickline which may permit limited data to be transmitted in real-time through the slickline to surface but which may still require at least some of the logged data to be recorded to a memory of the tool. After recovery of the tool to surface, the recorded sensor data is later associated with depth data determined from line measurements made at the surface over the same measurement time period. The association of the sensor data and the depth data is accurate provided the tool moves at the same speed as the line at the surface. However, due to the combination of line elasticity and irregular tool motion caused by friction between the tool and the well bore wall (and possibly also between the line and the well bore wall), the instantaneous speed of the tool may differ substantially from the speed of the line at the surface. Accordingly, it is known to correct tool depths determined from unspooled line length measurements made at surface according to measured tool acceleration. Tool acceleration may, for example, be measured using an accelerometer within the tool that has a measurement axis aligned with a direction of travel of the tool along the wellbore. Tool acceleration measurements may also be combined with line tension measurements to correct tool depths determined from unspooled line length measurements made at surface. However, such known methods of tool depth correction may still lead to inaccurate tool depth estimates. Where interpolation is used to estimate a measured property value at a particular tool depth, this may also lead to inaccurate estimates of the measured property at a particular tool depth.